The invention is concerned with a form of photosensitively colorable glass known as polychromatic glass. This is a transparent glass capable of having developed therein a range of colors that essentially spans the color spectrum. Color is developed in a polychromatic glass by certain combinations of radiation exposure and thermal treatment. Variation of color within a given glass results from variation in the initial exposure flux which is defined as the product of radiation intensity and exposure time.
As originally developed in the 1940's, photosensitive glasses were monochromatic, that is capable of having one characteristic color developed therein. Such a glass was so constituted that it underwent a change when exposed to short wave radiation, such as ultraviolet radiation. As a consequence, irradiated areas were capable of heat developed coloration, while non-irradiated areas remained substantially unchanged by such heat treatment.
U.S. Pat. No. 4,017,318, granted Apr. 12, 1977 to Pierson and Stookey, describes the general characteristics of a polychromatic glass, a particular type of such glass, and a procedure for developing a range of colors in such glasses. The polychromatic glasses disclosed in that patent may have a wide range of base compositions. However, each must contain silver, an alkali metal oxide (preferably Na.sub.2 O), fluoride, and at least one halide selected from the group of chloride, bromide, and iodide. The procedure includes exposures to high energy or actinic radiations selected from the group of high velocity electrons, X-radiations, and actinic radiations in the ultraviolet portion of the spectrum, preferably in the range of about 2800 A-3500 A. These exposures are followed by heat treatments at temperatures between the transformation range of the glass and the softening point thereof.
More specifically, in the Pierson-Stookey procedure, the glass is initially exposed to high energy or actinic radiation to photoreduce silver and thereby develop a latent image in the glass. This exposure determines the final color which will be produced in the glass. Thereafter, the glass is subjected to a heat treatment which causes the formation of colloidal silver particles in situ. These act as nuclei for the growth thereon of extremely small microcrystals of alkali metal fluoride and a mixed alkali-silver halide, e.g., NaF+(Na,Ag)Cl and/or (Na,Ag)Br and/or (Na,Ag)I. The nucleated glass is then cooled--conveniently to room temperature but, in any event, to a temperature at least 25.degree. C. below the strain point of the glass--and again exposed to high energy or actinic radiations. This second exposure intensifies the ultimate color, the hue of which was previously determined via the first exposure. Finally, the glass is reheated to a temperature between about the transformation range and the softening point of the glass. This produces the desired colors in the glass by deposit of metallic silver as discrete colloidal particles and/or deposited on the surface and/or within the alkali metal fluoride-mixed halide microcrystals.
The quantity of silver precipitated, and the geometry thereof, as well as, perhaps, the refractive index of the crystals, are deemed to determine the color produced. However, the colors can be achieved with very low silver contents and exhibit characteristics similar to interference colors. Hence, it was surmised that at least one of the three following circumstances is present: (1) discrete colloidal particles of silver less than about 200 A in the smallest dimensions; (2) metallic silver deposited within alkali fluoride-mixed halide microcrystals, the silver-containing portion of the microcrystals being less than about 200 A in the smallest dimensions; and (3) metallic silver deposited upon the surface of said microcrystals, the silver-coated portion of the microcrystals being less than about 200 A in the smallest dimension.
The patent further teaches that a sequence of colors may be developed by progressively increasing the magnitude of the exposure flux in the initial exposure step. Thus, progressively increasing the radiation flux (by increasing intensity, time, or both) successively produces pale yellow, green, blue, violet, red, orange, and dark yellow colors.
U.S. Pat. No. 4,092,139, granted May 30, 1978 to J. Ference, discloses an alternate color developing procedure. In this procedure, the second exposure (the reexposure) of the Pierson-Stookey method is carried out at a temperature between 200 and 410.degree. C. to cause the metallic silver to precipitate. This obviates the separate reheating step.
U.S. Pat. No. 4,134,747, granted Jan. 16, 1979 to Pierson and Stookey, describes a procedure wherein the potentially polychromatic glass is melted in a reducing atmosphere. During the color development procedure, this modification produces a so-called reverse opal effect. Exposed portions of the glass remain transparent, although colored, and unexposed portions of the glass become opacified.
The earlier Pierson-Stookey U.S. Pat. No. 4,017,318 indicates that the exact reaction path leading to coloration is not known with certainty. However, a proposed explanation is that silver nuclei are photosensitively precipitated. Cerous ions are used as a photosensitizing agent, as illustrated by this equation: Ag.sup.+ +Ce.sup.+3 +hv .fwdarw.Ag.degree.+Ce.sup.+4. The nucleation and growth of complex silver plus alkali metal (Na) halide crystals is then proposed as follows: ##EQU1## C is a crystallite of the nature (Ag.degree.).sub.n x(Na.sup.+ +Ag.sup.+)X and X.sup.- is a halide ion. The development of the colored species, resulting from a second radiation step and second heat treatment, is then expressed as: ##EQU2## CmAg.degree. is a crystal having granular particles of metallic (m) silver formed on or dispersed within it.
The patent provides a photomicrograph illustrating the acicular morphology observed in crystals present in the glasses following coloration. It points out that such crystals are believed to consist of a complex alkali fluoride-mixed halide combination which crystals may differ from the conventional cubic structure of the alkali fluoride. Thus, the complex crystals observed in the colored polychromatic glass exhibited the shape of elongated pyramids with length-to-width ratios greater than two. This degree of anisotropic morphology can vary widely, depending upon the base composition of the glass and the heat treatment applied thereto. The blackened tip of the crystal represents silver concentrated in or on the tip of the crystal. In the colored glasses prepared according to the general method described above, the crystals will be randomly oriented.
In summary then, photosensitive glasses, as orginally developed, were monochromatic and were developed by a combination of one exposure to ultraviolet radiation plus one heat treatment. A unique characteristic of polychromatic glass is its capability of having developed therein a range of colors through a combination of two radiation exposures and two heat treatments. In terms of crystal development, the polychromatic glass color development has been ascribed to an initial reduction of a small amount of silver as nuclei. This is followed by growth thereon of a complex alkali fluoride-mixed halide, pyramidal-type crystal. Ultimately, photoreduction of silver occurs with precipitation at the extremities of the halide crystals grown on the original silver nuclei.